Bump third_party/openthread/repo from `f0b6fce` to `a147485` #905

Job	Run time
Build on Darwin (clang, python_lib, simulated)	40m 28s
Build on Linux (fake, gcc_release, clang, simulated)	2h 7m 47s
Build on Linux (coverage)	16m 14s
Build on Linux (gcc_debug)	1h 11m 50s
Build on Linux (python_lib)	31m 53s
Build on Linux (python lighting-app)	10m 34s
	4h 58m 46s

Provide feedback